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Tiny Transistors Scout for Cancer

NEW ORLEANS--Nanoscale electronics experts are often ribbed for making circuitry that microchip companies already make millions of times better. But at the American Chemical Society meeting here yesterday, researchers described a nanoscale device that does something microchips can't touch: tracking proteins indicative of two different forms of cancer. Down the road, arrays of such sensors could allow doctors to instantly screen patients for multiple diseases.

Researchers have been experimenting with electronic diagnostics for years, but with mixed success. The detectors typically use field effect transistors (FETs), devices in which a voltage applied to one electrode, called a gate, changes the current flow between two other electrodes. To make a diagnostic, researchers typically replace the gate with a material coated with proteins or other compounds designed to capture molecules of interest. As long as the captured molecules are charged, their presence changes the conductance between the transistor's two electrodes. But although such devices often work, they are typically not very sensitive, because they require a huge number of target molecules to bind.

In hopes of improving the sensitivity, Harvard University chemist Charles Lieber and his students tried the same setup with nanoscale transistors. Lieber's group has made such transistors for years, typically using silicon wires as small as 1 to 2 nanometers across. For this study they fashioned gates from silicon nanowires coated with antibodies for prostate-specific antigen (PSA), a marker for prostate cancer. They then positioned the transistors under a plastic pad patterned with tiny channels that allowed fluid to flow over them.

When the team injected a dilute solution containing PSA, the negatively charged protein bound to the antibody and altered the conductivity of the FET, Lieber's graduate student Wayne Wang reported at the meeting. In fact, he says, he and his colleagues managed to detect PSA down to levels of 0.025 picograms per milliliter, making it the most sensitive PSA detector yet created--more than 100 times as sensitive as commercial PSA tests. Wang also reported that a similar sensor picked up the presence of CEA, a marker for colorectal cancer. And a simple array of nanosensors detected both cancer markers simultaneously.

"They have made really beautiful progress," says Jie Liu, a chemist at Duke University in Durham, North Carolina. "The question is, can these be made reliably and cheap in large quantities?" Nanowire sensors, he says, still have a long way to go before reaching the market. But if successful, they could provide doctors with an instant readout of whether a patient has a wide variety of diseases.